28-day test failed - okay to wait for 56-day?

[StructureMan44]

The results of the 28-day tests aren't meeting the target compressive strength. We believe it's due to the strength retarding properties of fly ash in the mix. Assuming the 56-day test meets the target; are there any implications to long-term durability (40 year service life) with utilizing concrete that met it's target strength late? The foundation won't be loaded until the target strength is met.

 

[TehMightyEngineer]

Nothing significant that I'm aware of. What's the strength you're shooting for?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[PEinc]

Also, what concrete strength is actually NEEDED?

http://www.PeirceEngineering.com

 

[DETstru]

Typically that's not an issue.

How far off are the breaks coming in at?

 

[StructureMan44]

Thanks everyone. The target strength is 5800psi; the 28day strength averaged at 93% of that. Surprisingly core samples taken a few days after the 28-day strength are coming up as low as 3790 which is 65% of target strength. Because of this we believe there is a large difference in the mix at the top of the foundation vs. the bottom of the 4'9" thick foundation. The 28 and 56 days samples are taken before any concrete is poured (to check for air in the mix) while the cores are taken at the end of the placement.

 

[dik]

Why 40 MPa? for strength or serviceability? Not likely an issue, but, you are not getting what you asked for. Why was it specified as such?

Dik

 

[PT99]

Did you watch the concrete placement yourself?
It is not unusual for the testing lab to do their tasks first, then the concrete crews proceed to work and ask for more water, without further lab testing.
Obviously then the samples do not reflect the actual installed concrete.

When this happens, the truck mix driver (at least here in the NYC area) will ask the foreman to sign the tickets to accept changes in the delivered mix design.
You could ask for copies of the tickets sent back to the supplier and see if this happened.

The other thing to consider is that test samples are cured under ideal ASTM specified curing conditions, while the cores reflect the actual curing conditions which are not the same.

In the end, I would put more trust in the cored samples than the test samples.

 

[TehMightyEngineer]

Ooof, if your cores are coming up at 65% of target strength at 28 days then I'd be concerned that you might not reach 5800 psi.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[bhiggins]

ACI defines acceptance criteria (ACI 14): 26.12.3.1

Looks like you are not meeting these requirements.

 

[dcarr82775]

You are not going to gain 35% between 28 and 56 days, fly ash or not. Further investigation is warranted to verify in place strength

 

[oldestguy]

Note the there may be the possibility of using the "wrong" fly ash.

Some fly as may actually decrease final strength.